Variable frequency transducer



Aug. 6, 1957 F. J. FRY ETA!- VARIABLE FREQUENCY TRANSDUCER 3Sheets-Sheet 1 Filed Aug. 24, 1954 FRANCIS .1. FRY BY WgLIAM .1 my

Zrr'rs Aug. 6, 1957 Filed Aug. 24, 1954 F. J. FRY ET AL VARIABLEFREQUENCY TRANSDUCER 3 Sheets-Sheet 2 IN V EN TORS.

FRANCIS .1 FRY WILL/AM J FRY ATT'YS Aug. 6, 1957 J, FRY EI'AL VARIABLEFREQUENCY TRANSDUCER Filed Aug. 24, 1954 3 Sheets-Sheet 3 74 FIG 5 74 6766 f g 5 75 r'fff-E ,4

i-i l! 73 2g IN V EV TORS.

FRANCIS J. FRY

United States Patent Ofiice 2,802,196 Patented Aug. 6, 1957 VARIABLEFREQUENCY TRANSDUCER Francis J. Fry, Champaign, and William J. Fry,Urbana, 111., assignors, by mesne assignments, to the United States ofAmerica as represented by the Secretory oi the Navy Application August24, 1954, Serial No. 451,992 12 Claims. (Cl. 340) This invention relatesgenerally to a transducer in which a mercury column of variable lengthis intimately coupled to a piezoelectric, electrically driven element sothat a unit-vibrating element is formed, the frequency of which may bemade resonant and is capable of being varied as desired within itsprescribed limits. The invention is more particularly described as asound-radiating area composed of a matrix in which a relatively largenumber of vibrating elements is employed.

More particularly, this invention relates to the novel construction andoperation of a transducer of relatively large size having a plurality ofpiezoelectric units, electrically connected together so that acontinuously variable resonant frequency characteristic is possible inthe opera tion thereof. In the present invention, the variable frequencycharacteristic has been realized without greatly increasing the sizelover that of a fixed frequency transducer of the same radiating area.

An important object of the invention is to provide a transducerconstruction for large size transducers in which a continuously variableresonant frequency characteristic is possible.

A further object of the invention is to produce a transducer in whichthe variable feature resulting in a dimensional increase occurs mainlyin a direction 'at right angles to the radiating area of the transducerwhich has this radiating area in one plane.

More particularly, the invention comprises a matrix arrangement of aplurality of piezoelectric elements in a relatively large squareradiating face which is attached to a shell holding a mercury columnwhose dimension is varied by means of a mechanically driven piston.

A further object of the invention is to provide a transducer arrangementin which the piezoelectric elements comprise a pattern of singleelements or a division thereof into two, three, or more pieces whichoccupy the same space as a single element but together are arranged in asymmetrical planar pattern, usually with the elements with the greatestnumber of pieces for each unit at the center of a rectangular'patt ern.This arrangement is to produce a desirable beam pattern of sound.

Still a further obiect of the invention is to provide a variableresonant frequency transducer system having a housing for supporting amercury column continuously variable in one direction with a drivemechanism for varying the mercury column length and an overall housingenabling the system to radiate into the liquid medium.

Still a further object of the invention is to provide an improvedmounting for the piezoelectric units and means for connecting themintimately and positively to a variable mercury column.

Other objects of the invention will appear in the specification and willbe apparent from the accompanying drawings, in which:

Fig. l is a sectional view of a transducer construction in accordancewith this invention;

Fig. 2 is a face view of a transducer element as shown in Fig. 1, alsoillustrating somewhat diagrammatically,

portions of one pattern of an arrangement of the piezoelectric elementsdividing some of the unit spaces into one or more pieces per unit, butproviding for a symmetrical arrangement of the elements in an entirerectangular plane unit;

Fig. 3 is an enlarged fragmentary view showing the construction andassembly of piezoelectric crystals in the supporting frame and theelectrical contact space for engaging the mercury;

Fig. 4 is a perspective view of the transducer piezoelectric assemblywith common electrical connections for the different units thereof;

Fig. 5 is a face view of a fragmentary portion of the piezoelectric unitshowing connections of the divided piezoelectric units; and

Fig. 6 is a fragmentary sectional view showing the electrical connectionof the parts shown in Fig. 5.

In this relatively large size sound transducer, which may be used forunderwater sound, the matrix arrangement presents a square radiatingface which is attached to a shell holding a mercury column having onedimension which is varied by means of a mechanically driven piston. TheWhole arrangement is enclosed in a steel tank with a sound rubber windowso that it can be used as an underwater sound radiating transducer.

Although the piezoelectric element used in this construction issynthetically grown ammonium dihydrogen phosphate (ADP) crystal, it isequally possible to apply the same technique to a system using anyelectrochemical vibrating element. This includes the magnetostrictivevibrating elements as well as the newer ceramic elements such as bariumtitanate.

Referring now more particularly to the drawings, a relatively largeunderwater sound transducer is herein described which has a squarecrystal array mounted in a face plate 10 located at the front of amercury housing which is also square, and the sides of which are fourpieces 12 of solid metal, such as steel, carefully finished and surfaceground on all of the exposed surfaces. These steel sides are paintedwith a suitable adhesive (Hysol 6020, Houghton Laboratories) in the areawhere they will come in contact when they are bolted and attachedtogether. These sides are bolted together and the adhesive left to setso that liquid tightness in the corners is assured. With thisconstruction, it is possible to make a housing with dimensionalvariations of less than of an inch. At the back of the housing is ametal plate 14 secured to the plates 12 and having gasket grooves 16 toset the gaskets therein which engage the edges of the plates 12 to makea fluid-tight joint. The face plate 10 is also provided with gasketgrooves 18 about the edges of the inner side which is secured to themercury housing to also make a fluid-tight joint.

Within the housing is a square piston 20 preferably formed with marginalflanges 22 on each of its four sides and the corners of the pistonsupported in the housing by guide rails 24 in each of the four cornersof the housing. At the center of the piston and tightly secured'therethrough is a hollow piston shaft 26 which extends through acentral opening 28 in the back plate 14 as a guide. The front pistonface is surface ground so that an accurately planed and movable face ispresented in the back of the entire crystal array which is mounted inthe plate 16 bolted to the front face of the square housing.

To provide the necessary acoustical decoupling of the mercury column inthe housing from the supporting chamber, a pressure-release material 30is applied to the walls of the housing and to the face of the pistonwhich preferably comprises a layer of balsa wood, approximately A inchthick, which is held accurately planar by gluing the wood to the steelwith a surface-ground plate resting upon the wood during the settingprocess.

The mercury housing is mounted in a solid casing, preferably made ofmetal plates and comprising a bottom 32 with supporting side rails 33upon which the mercury housing is rigidly supported at the sidesthereof. At the front of the casing directly in front of the face plateis a front plate 34 having a window aperture 35 therein about the samesize as the face plate. Extending over the outside of the windowaperture and overlapping the outer edges thereof is a sound-transmittingwindow 36 of rubber or other suitable material which is tightly securedabout all of the edges of the window aperture by a marginal clamp 37suitably secured in place by fastening bolts 38.

At the rear of the casing is a back plate 39 with a discharge opening 40having a valved pipe extending therefrom at the bottom thereof and atthe sides are side plates 41 which extend upwardly to a common levelwith the front plate 34 where they are covered by a top plate 42. Atubular extension 43 is suitably secured to the top of the casingsurrounding the openings therethrough by a flange 44 and fastening bolts45. The top of this tubular extension 43 may be sealed by a threaded capor in any well-known manner, and fluid seals may extend through the topplate 42 into the extension to be accessible therethrough when the capis removed. Extending through the top plate 42 is a fill pipe 46 havinga control valve 47.

To reciprocate the piston in the mercury chamber, a hollow sleeve 48 isattached to the rear plate 14 extending rearwardly therefrom andsurrounding the central opening 28. One end of a threaded shaft 49extends within the hollow piston shaft 26 engaging internal threads 50thereof, and the outer end of the threaded shaft 49 is connected to agear 51 by a key 52 disposed between supporting plates 53 and 54 whichare secured to the outer end of the sleeve 48 allowing free movement ofthe gear 51 and the shaft 49 in the rotary direction but confining thelongitudinal movement of the shaft. A gear train 55 is connected to thegear 51 and is suitably supported by the mercury housing with a terminaldrive shaft 56 extending through the top plate 42 and sealed in itspassage therethrough by a packing gland 57 so that the outer end of theshaft projects into the extension 43 where it may be engaged by a toolA, similar to a socket ring or any other suitable means for turning theshaft and the gear train. The rotation of the shaft 56 in one directionor the other correspondingly moves the piston 20 at the back of thefront plate 10 thereby varying the mercury column at the back of thefront plate. By maintaining a fluid-tight connection through the top ofthe casing, the pressure in the casing may be varied as desired. Thismay be controlled through the inlet pipe 46, and the fluid usuallydeposited in the casing is degassed which completely fills the casingsurrounding the mercury housing.

In order to maintain a supply of mercury in the chamber in the mercuryhousing, an inlet opening 58 is provided in the top plate 12 with apassage 59 extending to the front and back of the chamber, and connectedto this opening is a pipe 60 which extends through the casing,preferably through the rear wall 39 thereof, with a suitable controllingvalve 61 at the outside of the casing. A similar connection may beprovided in the bottom plate 12 comprising an outlet opening 62 having apassage 63 extending to the front and rear of the mercury chamber. Tothis opening a pipe 64 is connected, which also extends through the rearwall 39 and is provided with a controlling valve 65. By this or anyother suitable means, mercury may be supplied to the mercury housing andwithdrawn therefrom if desired. In pipes 60 and 64, couplings areprovided so that the mercury housing can be removed conveniently fromthe outer chamber.

The front plate 10, which is attached to the mercury housing, comprisesa steel matrix having a plurality of partitions 66 forming rectangularor square cells in which piezoelectric crystals 67 are mounted. Thesecrystals are adhesively connected at the rear side of the plate to asheet 68 of silver palladium alloy (comprising 60 percent Ag and 40percent Pd), which is supported by the matrix plate and its partitions.This alloy sheet 68 is of the order of .010 of an inch thick and is spotwelded to the plate 10 and its partitions in a plurality of places toproduce an extremely well-knit structure. The alloy sheet 68 providesthe necessary intimate electric coupling between the crystals and themercury column. The coupling is obtained by amalgamating the alloy withthe mercury on the side opposite the glued crystals.

After completing the above assembly of the plate 10 with partitions 66and alloy sheet 68, the back surface of the alloy sheet is thoroughlycleaned with a scouring powder in preparation for electroplating ofcertain areas. The electroplated area covers the alloy sheet 68 andoverlaps the marginal edge thereof onto the back of the front plate 10except in the regions immediately behind where the crystals are glued. Athin layer of nickel 69 of the order of .0005 of an inch is depositedleaving the regions immediately behind the crystals for contact andamalgamation with the mercury. The regions with the nickel coating willnot amalgamate, neither does the mercury show a creeping tendency underthe nickel. An adhesive fillet 70 is provided between the edges of thealloy and of the front plate 10 as clearly shown in Fig. 3. This filletassures liquidtightness of the alloy sheet.

A paper mask 71 is applied by glue or another adhesive over the regionsin which the nickel coating has been electroplated. This paper acts asan acoustical decoupling material, and about .0015 of an inch inthickness is taken up by the nickel glue and paper layers. The adhesiveHysol 6020 previously mentioned is satisfactorily used for these gluingoperations with a room temperature set.

The piezoelectric crystals of various numbers of plies are glued intothe matrix plate 10 between the partitions with a suitable adhesive(Hysol 6020) as shown more clearly in Figs. 2 and 3. During theadhesive-setting period, the crystals 67 are held in position spacedfrom the plate 10 and the partitions 66 thereof by balsa wood strips 72.After the crystals are set, these strips are removed and a decouplingmaterial 73 (Hycar ebonite) is installed. While positioning thecrystals, the adhesive layer which connects them to the alloy sheet 68is closely observed and worked so that no visible bubbles remain betweenthe crystals and the alloy sheet. The crystals are provided with goldelectrodes and tabs 74 *0 which silver conductor wires 75 are solderedand connected. In the slots between the crystals and the steel matrixand its partitions, the balsa wood strips may be replaced by theacoustical decoupling material 73, and in the region between thecrystals above the steel plate, the same material is installed. Thismaterial serves to acoustically decouple the crystals from steel matrixand from each other.

When the electrical connections between the crystals and the conductorwires 75 are completed, the exposed alloy surface of the sheet 68 isamalgamated with mercury. The amalgamation with mercury is acceleratedby the use of a very small quantity of nitric acid which is wiped on thealloy plate and then wiped off by a slight scraping action of a knifeblade over the various surfaces. A good amalgamation is one on whichlint-free material can be used to wipe off the surface mercury, and thenupon application of a small drop of clean mercury, there is immediatecovering of the entire amalgamated region with a bright and shinymercury film. In a complete set-up of the crystal assembly, thematerials which come in contact with the mercury are steel, cast iron,balsa wood, Hysol adhesive, nickel, the paper mask, the silver palladiumalloy, and the gasket material, such as neoprene on the front and backplates and in the piston drive mechanism, the tubings 60 and 64 whichmay be plastic material, and the outside valves which enable the mercuryto be placed into the mercury housing after the completed transducerassembly. Materials such as zinc and alumimun must be avoided because oftheir contaminating influence on the mercury.

The crystals in the cells of the matrix may be in the form of singleunitary crystals 67, or the crystals for each cell may be divided intotwo portions 67a with suitable separating insulation 76, or they may bedivided into four insulated parts 67b, each connected by suitableelectrodes, usually arranged in a pattern as suggested in the upper leftquadrant but extending over the entire matrix in Fig. 2 in which four ofthe solid units 67 are at each corner, then two line crystals 67a extendin the two outer rows between each corner, and the four section crystals67b are in the thirty-six central cells. In this entire crystal system,the individual crystals or the divided parts thereof filling each cellare connected by the gold electrodes 74 either to a conductor wire 75 orto an opposite conductor wire 77. These wires are supported in a raisedposition above the crystals by supporting posts 78 as shown in Fig. 4and connected to common conductors 79 and 80. These conductors 79 and 80extend through a fluid-tight connection 81 in the top plate 42 of theconductor casing and into the space in the tubular extension 43. Thisarrangement is for crystals in which the voltage is applied at rightangles to the utilized direction of motion. There are many ways ofconnecting the various crystal elements and the visible portions thereoffor obtaining ditferent beam patterns.

In the piston 20, a plurality of openings 82 may be provided, and themarginal flanges 22 are spaced from the sides of the mercury housing toprovide for a quick passage and transfer of mercury in the mercurychamber from one side of the piston to the other in varying thefrequency of the transducer.

For this type of transducer, a relatively slow speed drive is providedhaving a total stroke of approximately 4 inches. The mercury housing iscompletely filled with mercury, and the casing enclosing the housing iscompletely filled with degassed castor oil. Sound is transmitted fromthe crystal assembly to the sound window 36. Pressure may be placed uponthe castor oil through the fill openings and also upon the mercury, ifdesired. In order to fill the transducer casing with castor oil underthe most favorable conditions, the casing is made air tight so that itmay be completely evacuated of air whereby the oil is pumped in oradmitted under vacuum. To accomplish this result, a solid metal windowwhich may be in the form of a plate 83 with a peripheral flange 84 maybe applied to the outside of the marginal clamp 37, and anair-exhausting pipe 85 may be inserted from the outside and connectedthrough a valve 86 With a suitable vacuum line. At the same time, vacuumconnections are made with the top of the transducer casing and with themercury housing through its pipe 60 since it is also emptied during theoil filling. With this technique, the hydrostatic pressures on bothsides of the sound rubber window 36 are approximately equal. Thedegassed castor oil is evacuated to the same pressure as the transducer,and the oil flows by gravity or by pumping it into the transducerhousing. After filling, the entire system is brought simultaneously toatmospheric pressure, and the valves leading to the castor oil regionare closed. The mercury is then introduced into the mercury chamberthrough the valve 65 and the pipe 64. The valve 61, which is theairexhaust valve for the mercury housing when it is being filled withmercury, may be closed when the mercury chamber is filled. After thehydrostatic pressure is equalized, the window plate 83 is removed andthe sound window 36 is free for sound vibrations.

To support the transducer beneath the surface of the water for testpurposes, the tubular extension 43 may be of a length desired for thedepth in the water. This pipe provides passage for the tool A used as aturning rod for changing the piston location in the mercury chamber andfor access to the terminals for the electrical supply conductors 79 and80. If the transducer casing is located sufliciently below the surfaceof the water to make a material change in the pressure applied to thesound window 36, a balancing pressure may also be applied to thetransducer casing through the fill pipe 46.

This transducer design is thus able to meet the requirements of sonar(underwater sound) apparatus for use on naval and other vessels. Forthis purpose, a remote drive mechanism for varying the mercury columnlength is used, together with means for controlling the pressure in thetransducer casing in the mercury housing as above described. The holes82 through the piston and the spaces at the edges of the piston areprovided for rapidly displacing mercury around the piston for adjustingthe movement thereof.

With this construction and the adopted arrangement of crystals, avoltage is applied at right angles to the utilized direction of motion,the column of mercury may be varied and has an intimate acousticalcoupling with a plurality of piezoelectric, electrically driven elementscapable of vibrating the variable resonant frequencies. The vibrationsthus produced will pass through the flexible sound window and may bethus projected for remote pickup. This action also may be reversed andvaried to receive sound through the transducer window which activatesthe piezoelectric units to produce an electric signal, the resonantfrequency of the units being varied by changing the mercury column.

While a preferred embodiment has been described in some detail, itshould be regarded as an example or embodiment of the invention and notas a restriction or limitation therein as many changes may be made inthe construction and arrangement of the parts without departing from thespirit and scope of the invention.

We claim:

1. A variable resonant frequency transducer comprising means forsupporting a mercury column variably in one dimension, an array ofpiezoelectric elements intimately coupled at their inner faces to thismercury column and exposed at the outer faces, means for varying saidone dimension of the mercury column, an overall fluid-tight casing forenveloping said supporting and varying means with an openingtherethrough having a flexible partition in the outer wall spaced fromthe said outer faces but in position for the array of elements toradiate therefrom, a liquid medium in the casing between the elementsand the partition, and electrical conductor means for supplying currentconnected to the elements and extending therefrom out of the casing.

2. In a transducer in accordance with claim 1, the array ofpiezoelectric elements comprising a metallic grid separated intoadjacent closely related cells and a plurality of piezoelectric unitsmounted in the cells with their corresponding ends in a common plane andmeans for confining the mercury column in contact with said ends in acommon plane.

3. In a transducer in accordance with claim 2, a housing for confiningthe mercury column, a piston having a hollow shaft extendinghorizontally from the housing and internally threaded, an externallythreaded shaft fitting within the hollow shaft, and a train of gearingconnected to the externally threaded shaft at the outside of the housingin said liquid and extending therefrom out of the top of the casing forvarying the said one dimension of the mercury column.

4. In a transducer in accordance with claim 3, the piston havingopenings therethrough through which mercury may move, the housing havingsupporting rails for the piston providing marginal spaces between thepiston and the housing through which mercury may pass from one side ofthe piston to the other when the piston is relatively moved in thehousing.

5'. A transducer in accordance with claim 4in which the mercury housinghas top .and bottom openings with passage extensions therefrom with thefront and rear of the housing and tubular supply connections to theopenings and leading therefrom through the overall casing with valvemeans at the outside of the casing for controlling the supply of mercuryand the pressure thereon in the housing.

6. In a transducer in accordance with claim 2, the piezoelectric unitsbeing both solid and having portions thereof which make up a unit andbeing mounted in individual cells of the plate spaced electrically fromthe partitions forming the plate and extending substantially flush withthe plate at one end but projecting from the plate at the other end, acommon alloy sheet of conducting metal attached to the ends of the unitsflush with the plate and over the adjacent edges of the partitions andthe plate surrounding the cells, the piezoelectric units and theportions thereof being spaced from the partitions and from the marginaledges of the plate, an insulating means disposed between and secured tothe outwardly projectends of the elements and portions thereof on top ofthe outer ends of the partitions.

7. In a transducer in accordance with claim 6, the outer exposed face ofthe common metal sheet and ad jacent edge of the supporting plate beingcovered with a thin electroplated layer of nickel except for the regionsimmediately between the crystal units and parts thereof, and a thin maskof acoustical decoupling material, such as paper, applied over theregions in which the nickel coating has been electroplated.

8. In a transducer in accordance with claim 7, the face of the pistonopposite the inner ends of the piezoelectric units being provided withan acoustical decoupling layer of material and the sides of the housingalso having a thin acoustical decoupling material applied theretothereby acoustically decoupling the mercury column into the housing fromthe supporting chamber.

9. In a variable frequency transducer, a housing for supporting amercury column, an array of piezoelectric elements extending in a planeat the front of the housing and the rear of the elements electricallycoupled to the mercury column, a piston movably mounted in the housingfor varying the effective mercury column at the back of the elements, anoverall casing for enclosing the housing having an opening therethroughin the path of radiation from said piezoelectric elements, a flexiblesoundtransmitting window fitting tightly in the opening, a shaftextending from the piston through the housing, a train of gearingconnected to the piston shaft and extending from the housing therefromthrough the top of the casing, electrical connections from thepiezoelectric elements also extending from the top of the casing, meansfor sealing the gearing and electrical connections at the top of thecasing, a sound-transmitting liquid in the casing surrounding thehousing, and sealed means extending through the casing for supplyingmercury under pressure to the housing.

10. In a transducer in accordance with claim 9, the casing and thehousing being liquid tight for keeping separate the liquids which theycontain, the casing having an extension secured to the top through whichthe gearing and the electrical connections extend in fluid-tightarrangement, and the casing having valved tubular means for controllingpressure and vacuum therein.

ll. In a transducer in accordance with claim 10, a removable plate orcover to fit over the outside of the opening in the casing and at thefront of the flexible sound-transmitting window and means for varyingthe pressure in the space at the outside of the window covered by saidplate and for equalizing the pressure at both sides of the window, theplate being removed when the transducer is in use.

12. In a transducer in accordance with claim 6, the top sealing meansforming an extension of the casing of sulficient length so that thetransducer may be located below the surface of the liquid in which it isimmersed, the train of gearing and conductors being accessible throughthe extension from above the top of the liquid, and supply and pressurepipes from the housing and casing also extending above the surface ofthe liquid in which the transducer is immersed so that pressure may beapplied thereto for equalizing the pressure within the casing tocorrespond with the external pressure of the liquid in which thetransducer is immersed.

References Cited in the file of this patent UNITED STATES PATENTS1,619,125 Hough Mar. 1, 1927 2,400,063 Barton et al. May 14, 19462,404,391 Mason July 23, 1946 2,417,830 Keller Mar. 25, 1947 2,521,642Massa Sept. 5, 1950 2,689,947 Fry Sept. 21, 1954

